This invention relates to lightwave systems and more particularly, to coherent lightwave systems.
In a conventional lightwave system the output of a laser or LED is amplitude modulated, for example, digitally, so that information to be tramsitted is contained in the intensitey nvelope of the lightwave signal. The frequency of the lightwaves, on the other hand, contains no such information, and in a conventional fiberoptic system, is related only to such transmission characteristics as attenuation and dispersion. In contrast, in a coherent lightwave system the phase or instantaneous frequency of the lightwaves must be well-defined and controlled in order to transmit information. For example, in a phase-shifted-keyed (PSK) coherent system the phase of the lightwaves carries the information, and in a frequency modulated (FM) system the instantaneous frequency of the carrier contains the information. In an amplitude modulated (AM) coherent system using heterodyne detection the precise frequency of the lightwaves, which serve as a carrier, is important because in the receiver the carrier signal is mixed with a local master oscillator signal to generate a lower frequency beat signal from which the information is extracted.
It is apparent, therefore, that the frequency of the light source is crucial to the successful operation of a coherent system and the realization of its primary advantage, extreme sensitivity (i.e., the ability to detect low intensity signals). For a given optical power the latter implies transmission over longer distances with fewer repeaters than a conventional lightwave system. It follows, therefore, that it would be desirable to maximize the optical power transmitted for a given amount of electrical bias power supplied to the transmitter of a coherent system. The transmitted power, in principle, could be increased by simply using a more powerful laser, but in doing so other system parameters must be carefully considered; for example, the power threshold for damage to the modulator use to impress information onto the lightwave output of the laser, and the loss experienced by the laser output in passing through the modulator.